The disposal of waste materials, especially of toxic waste, with plasma torches is well known and has been performed for some time. In such a process, a plasma torch transfers electrical energy through a stream of ionized gas so that the gas becomes an electrical conductor. With such a torch, very high temperatures of as much as 10,000.degree. C.-15,000.degree. C. can be attained.
In general, there are two types of plasma torches, non-transferred arc torches, in which the electric potential is entirely contained within the torch, and transferred arc torches, in which an arc is struck between the body of the torch on the one side of an electrical field and a point or area spaced therefrom. The present invention is particularly useful with transferred arc torches.
Generally speaking, a plasma torch disposal reactor raises the temperature of waste materials, including toxic waste materials, to such high levels that they chemically break down (pyrolysis). This breakdown can be enhanced by maintaining an atmosphere of the appropriate gas in the incinerator. As a result, the residues are usually harmless gases and solids which can be suitably removed from the incinerator.
In the recent past, significant improvements to such incinerators or reactors were made. The commonly owned U.S. Pat. No. 4,912,296, for example, discloses an advantageous construction for a plasma torch useable with incinerators of the type here under consideration. U.S. Pat. Nos. 4,770,109 and 5,136,137, both by the inventor of this application and also commonly owned, disclose and claim reactors for the incineration and melting of all types of materials, particularly hazardous waste, with which the invention of this application is particularly useful. The disclosure of U.S. Pat. No. 5,136,137 is herewith incorporated herein by reference.
Other patents relating to the field of materials incineration and melting include U.S. Pat. Nos.:
______________________________________ 3,599,581 4,432,942 3,779,182 4,582,004 4,181,504 4,615,285 4,326,842 4,918,282 ______________________________________
and British patent 1,170,548.
The reactor disclosed and claimed in the '137 patent incorporated herein by reference uses a rotating, material-receiving drum or chamber into which the hot plasma of a plasma torch is directed. The inner surface profile of the rotating drum is appropriately shaped and constructed so that by varying the speed of rotation of the drum, materials placed into it are spread out over the inner surface of the drum to form a relatively thin layer of such materials which has a large surface area and which can therefore be brought more quickly to the desired high temperatures generated by the plasma torch.
A critical element of such reactors is the rotating drum. It is an open, upright drum that rotates about a vertical axis and has a drum base from which an upright, usually cylindrical outer drum wall extends. The center of the drum forms a discharge opening through which incinerated and melted materials can be gravitationally withdrawn from the drum. The '137 patent discloses in detail how the material in the drum is melted and how the drum is constructed and operated for withdrawing the molten material.
When operating the reactor with a transferred arc-type plasma torch, an electrical conductor must be placed at the bottom of the drum. Current then flows from the torch (which is suspended into the drum from the surrounding containment housing) to the conductor at the bottom of the rotating drum. In the past, a special, electrically conductive throat ring was provided which defined the discharge opening for melted material from the drum and which was appropriately electrically connected (grounded) so that the plasma arc can be generated.
Because of the extreme temperatures inside the drum, the throat ring had to be effectively cooled. In the past, this was done by constructing it of copper and appropriately surrounding its periphery with coolant passages, all as is disclosed in the '137 patent, for example. The necessary cooling not only caused significant heat losses from the reactor to the coolant, it also had the tendency of cooling and eventually freezing molten slag formed by the materials and accumulating at the bottom of the drum. Molten slag is electrically conductive at the high temperatures attained in reactors and, therefore, provides a path for the current flow from the torch to the throat ring and hence to ground. Frozen (solidified) slag, however, becomes an insulator. Thus, the necessary cooling of the throat ring could lead to the interruption of the current path when the slag freezes.
The construction and operation of such reactors, and in particular of the required throat ring, was subject to two antagonistic requirements. The first is to maintain the throat ring sufficiently cooled so that it is not damaged by the high temperature prevailing inside the drum and especially in the vicinity of the discharge arc between the throat ring and the torch. This, however, can lead to slag solidification if the cooling is not carefully controlled and limited. The second requirement is that the cooling of the throat ring should be limited to prevent slag solidification, but the resulting higher temperatures to which the ring is exposed could damage it.
In addition, prior art incinerating and melting furnaces of the type discussed above required substantial amounts of coolant flow. This required relatively intricate and complex coolant flow patterns in the drum, especially in the vicinity of the copper throat ring, and the provision of difficult to seal and maintain, large diameter rotating coolant water seals. Such furnaces, therefore, were not only relatively costly to construct, they required extensive and careful maintenance. Their production costs were further increased by the need to maintain precise concentricities in the mounting and sealing of the drum with respect to the containment housing. All this added to the overall cost of installing and maintaining such reactors and shortened their service life.